Heat-resisting steel

ABSTRACT

A heat-resisting steel with superb heat resistance, and yet with a low coefficient of heat expansion, against such hightemperature environments as above 900*C is obtained by uniformly dispersing 2 - 10 percent by weight of one or more carbonitrides selected from the group of those of titanium, zirconium, niobium, tantalum and vanadium into a steel containing 1 - 30 percent by weight of chromium.

United States Patent Abe 1 Oct. 1, 1974 1 HEAT-RESISTING STEEL 2,080,0015 1937 Becket 75/126 F 2,232,705 2/1941 Hull [75] Inventor: YoshihikoAbe, Tokyo, Japan 2,268,427 12/1944 Schlumpt 73] Assignee; i i hi SeikoKabushiki Kaisha 2,370,124 2/1945 Charlton 75/126 F Tokyo Japan2,801,916 8/1957 Harris 2,848,323 8/1958 Harris 75/126 P [22] Filed:June 5, 1972 [21] Appl. No.: 259,541 Primary Examiner-Hyland BizotAttorney, Agent, or Firm-Wenderoth, Lind & Ponack [30] ForeignApplication Priority Data July 20, 1971 Japan 46-53592 [57] ABSTRACTMar. 6, 1972 Japan 47-22245 A heatqesisting Steel with Superb heatresistance and [52] US Cl. 175/124 75/125 75/l26 C yet with a lowcoefficient of heat expansion, against 35/126 i 75/126 suchhigh-temperature environments as above 900C [51] Int CL 6 39/54 isobtained by uniformly dispersing 2 10 percent by [58] Field P 126 Fweight of one or more carbonitrides selected from the group of those oftitanium, zirconium, niobium, tanta- [56] References Cited lum andvanadium into a steel containing 1 30 per- UNITED STATES PATENTS cent byweight of chromium. 1,542,233 6/1925 Girin 75/128 T 1 Claim, 2 DrawingFigures TENSILE STRENGTH (Kg /mm G TENSILE STRENGTH INCREASE IN WEIGHTDUE TO OXIDATION (800Cx30hr) (rng/cm INCREASE IN WEIGHT DUE TO OXIDATIONAMOUNT OF CARBONITRIDE (WEIGHT /o) HEAT-RESISTING STEEL BACKGROUND OFTHE INVENTION The present invention relates to heat-resisting steels,and more particularly to steels with superb heat resistance, and yetwith a low coefficient of heat expansion, against such high-temperatureenvironments as above 900C, especially containing sulphur.

In general, heat-resistivity is an indispensable condition for steels tobe used in high-temperature circumstances; and beside great strength andtoughness not only against high temperatures, such steels are desired topossess great resistance against various hightemperature atmospheres,and yet with a low coefficient of heat expansion.

SUMMARY OF THE INVENTION A principal object of the present invention isto provide a heat-resisting steel which satisfies all of the desirablecharacteristics abovementioned.

It is another object of the present invention to provide aheat-resisting steel which has a heat-resisting quality comparable tothat of the conventional heatresisting steels and yet has such anexcellent resistance to sulphur as not being possessed by theconventional heat-resisting steels.

It is a further object of the present invention to provide aheat-resisting steel which, in comparison with the orthodox ones, issuperb in heat resistivity, and particularly so against sulphuricmatters, while its coefficient of heat expansion is remarkably small.

Another object of the present invention is to provide a heat-resistingsteel whose heat resistivity does not rely on such an expensive elementas nickel and which chromium in a considerably small amount incomparison with its amount for the orthodox ones, still being providedwith superb heat resistance.

A steel according to the present invention is characterized by achromium containing steel having specific carbonitrides dispersedtherein uniformly and finely beyond certain degree of concentration.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects of the presentinvention will become more apparent to those skilled in the art whenconsidered in reference to the following detailed description in thelight of the accompanying drawings wherein:

FIG. 1 is a photograph showing the microstructure of a steel madeaccording to the present invention in which 4.7 percent by weighttitanium carbonitride is dispersed into a steel mainly consisting of, byweight, 24.5 percent chromium, 3.5 percent molybdenum and 1.7 percentsilicon; and

FIG. 2 is a diagram showing the tensile strength at 700C of varioussteels and also the weight increases due to oxidation when they areexposed in the air for 30 hours at 800C, whereby the steels are those inwhich, in to the steels mainly consisting of, by weight, 1.0 percentchromium and 0.25 percent molybdenum, various amounts of such sole orcomplex carbonitrides as titanium, titanium niobium or zirconiumvanadium are dispersed.

DETAILED DESCRIPTION OF INVENTION In the manufacture of theheat-resisting steels of the present invention, it is necessary,firstly, to add carbonitride-forming elements to a molten steel preparedbeforehand containing a remarkably excessive amount of carbon incomparison with the amounts of oxygen, nitrogen, and sulphur, so as tocause a preferential reaction between the carbonitride-forming elementsand the carbon, and then to uniformly disperse and precipitate thethus-obtained solid carbide together with the nitride, and lastly it isnecessary for the resulting dispersed carbonitrides to be stable eventhough it is subject to high temperatures.

As a carbonitride-forming agent, which achieves such auniform dispersionand thermostability of the solid carbonitride in the microstructure ofthe steel, a carbonitride may be named with such a composition of one ormore elements selected from among such elements as titanium, zirconium,niobium, tantalum and vanadium.

In these carbonitride dispersing agents, the size, uniformity, andquantity of a dispersant are important factors to determine theircharacteristics, so it has been confirmed that a preferentialnitridation of the boron in the molten steel is desirable for thisreason.

From the explanation above, it is easily understandable that the steelswhich can meet the requirements of the present invention can bemanufactured by an industrial mass production system.

FIG. 1 shows a microstructure of an example of the heat-resisting alloysteels according to the present invention which clearly shows theunifrom distribution of the carbonitrides in the steel.

The effect of a carbonitride uniformly dispersed in the steel on itsquality at high temperatures will be explained in reference to FIG. 2.There are shown in it the variations of the steelshigh-temperature-proof strength (at 700C) and high-temperature oxidationresistance (800C X 30 hr) in the case of the various carbonitridescomposed solely or complexly of titanium, or titanium-niobium, orzirconium-vanadium dispersed in a range of up to 10 percent by weight.Irrespective of the kind of the dispersing carbonitride, the respectivecharacteristic curves can be plotted by an identical curve. However,with the amount of the carbonitride being below 2 percent by weight itscontribution toward the heat-resistivity of the steel is almost zero;however, if the its content surpasses that value, it sharply increasesthe heat-resistivity of the steel. In the case of the content of thecarbonitride being above 7 percent by weight, its effect remains nearlyunchanged, so it is understandable that its preferable content should belimited to less than 10 percent by weight, viewed from the aspects ofmanufacturing technique and economy.

In this case, the components of the matrix having the dispersion ofcarbonitrides cannot be ignored. Table I indicates the comparativeeffects of the components of a matrix of steel having the dispersion oftitanium or zirconium carbonitrides in 4 percent by weight or so,particularly the comparative effect of the amount of chromium dispersedin the steel, in terms of vanadium attack resistance and oxidationresistance. As is apparent from Table I, the steels which contain nocarbonitride and that in which the carbonitride is dispersed into thematrix belonging to the carbon steel (the sample Nos. 1 3) havepractically no vanadium attack resistivity (refer to the column of theV-attack resistance") and a little oxidation resistivity (also refer toOxidation resistance,) while the steels according to the presentinvention (sample Nos. 4 and in which chromium is contained in thematrix clearly show that In the steels according to the presentinvention, their corrosion resistivity at high temperatures areremarkably improved by the existence of one or more such elementscontained in the matrix as silicon, molybdea high temperatures theirresistance to corrosion 5 num, boron, copper and aluminum as well as thecarsharply increases. From this result, it is concluded thatbonitride-forming elements such as titanium, zircoin a steel to be madea matrix having the dispersion of nium, niobium, tantalum and vanadium.In order to a carbonitride, it is essential to add chromium. Thus itshow this fact, in Table 3, there is shown the influence can be sa1dthat the feature of the present invention lies of the chemicalcompositions of the steel according to 1n making advantage of thesynergistic effect on the the present invention on its oxidationresistivity at high heat res1st1vity of the steel of the dispersedparticles of temperatures. In Table 3, the steels belonging to thecarbonitrides and chromium contained in the matrix. group of samplesmarked A are those consisting mainly Table 1 Dispersed carbonitride Mainalloy V-attack Oxidation components resistance resistance Sample in thematrix (loss in (increase in No. Kind Amount(weight weight; weight:

mg/cm lhr) mg/cmlhr) 1 l.0%Cr-0.25%Mo 195 1.01 2 240 1.30 3 Ti 4.6 2320.90 4 Ti 4.3 1.1%Cr 132 0.26 5 Zr 3.8 1.0%Cr-0.5%Mo 145 0.18

Oxidation resistance The test piece is exposed in the air at 800%. forhours.

Then, in reference to Table 2, an explanation will be given on theeffect of the quantity of chromium to be added into the steel of thepresent invention. In reference to steels containing chromium indifferent amounts ranging 1 to percent by weight having a dispersion of5 to 6 percent by weight titanium-tantalum carbonitride, their oxidationresistivity and sulphurousacid-gas-proofness were compared at varioustemperaweight chromium for 1,100C to endure the effect of the presentinvention, and it can be understood that if the chromium concentrationis ensured so much as that, the steels of the present invention have aphenomeral heat-resistivity.

of 25 percent by weight chromium and 8 percent by weight nickel having adispersion of the carbonitride of titanium and it is understood thattheir oxidation resistivity is remarkably improved by the compositeaddition of molybdenum, boron and aluminum. Then, the 'steels belongingto the group of samples marked B are those consisting mainly of 18percent by weight chromium with an addition of one selected from amongmo lybdenum, aluminum and copper with carbonitrideforming element addedabove the concentrations necessary for the formation of the dispersedparticles so that they are also dissolved in the matrix, and they showthat their oxidation resistivity is remarkably improved. The steelsbelonging to the group of samples marked C, which consists principallyof chromium, are shown to compare the effect of the addition of theelements abovesaid with the effect of addition of silicon, and, it isunderstood that the addition of these elements has the effect to reducethe necessary amount of chromium which is determined by the kind and thetemperature of the environment gases, and silicon is especiallyeffective with respect to this point.

Table 2 Increase in weight Increase in weight s l by nxidatinn (mg/cm bvsulphurous acid as m cm Dispersing amounts Amount of 900CX10 hr1.100CX10 hr 800C l0hr 910C 10hr 1.000CX5hr chromium of Ti-carbonitridein the matrix (in the air) (in the air) (in 1OO%SO (in %SO,) (in 100%SOweight I Table 3 Chemical composition Oxidation resistance SampleDispersed mark particle Matrix Oxidation rate (mglcm /hr) Kind Ci NI SiMo Al Cu B others 1200C 1100C 1000C 1 Ti 4 5 25.2 8 3 10.8 2.52 0.13 A 2Ti 5 25.5 8 0 2.1 0.015 1.0 0.22 0.04 3 Ti 4 1 25.4 8 2.0 1.0 0.4 0.100.04 1 Ti 50 18.1 1.31 1.05

B 2 Ti+Nb 6.7 18.3 2.1 1.05 0.42

Zr 3 Zr 54 17.6 3.8 3.7 0.20 0.057

Ta 4 Ti+Ta 3.8 17.3 1.3 1.2 0.03 0.030

V 5 Ti+V 4.3 18.1 1.9 2.6 0.15 0.082 1 Ti 4.5 26.9 3.10 0.52 0.17

Ti 2 Ti 4.3 24.5 0.8 0.35 0.17 0.09

Ti C 3 Ti 4.1 20.0 4.2 2.1 0.20 0.10 0.05

Ti 4 T1 3.8 18.2 1.1 3.1 1l 4 0.27 0.10 0.027

Ti 6 Ti 4.1 18.0 1.7 1.0 1.0 0.32 0.12 0.07

In this connection, it will be appreciated that the kinds of theadditional elements vary depending on the objects of use of the steels,and, in the case where the high strength at high temperatures is not somuch needed, even steels in which ferrite constitutes its matrix can dowell, and as for the amounts of the elements to be added into the steel,when, at relatively low temperatures, the total amount is 0.2 percent byweight or more, its effect comes to display. However, as the temperaturerises, the total amount needs in increase.

When 1,000C is made the object of the steels highheat resistivity, thetotal amount must be made 8 percent by weight or so as shown in Table 3.

Besides, with high-heat-resisting steels, it is desirable to make itscoefficient of heat-expansion small when exposed to a high-temperatureenvironment; however, with the steels of the present invention, sincethe coefficient of heat-expansion of the carbonitride to be dispersedtherein is hear half that of the steel of the matrix, the coefficient ofheat-expansion of the steel of the present invention, in proportionthereto, is expected to become less in comparison with that of theorthodox steels. Table 4 shows the coefficients of heat-expansion at to600C of the steels of the present invention in comparison with that ofthe orthodox steels, which reveals that with the steel of the presentinvention, its coefficient of heat expansion is lower than itscalculated value predicable on the basis of the volume ratio of thematrix and the dispersed particles. That is, in the steels according tothe present invention, the dispersed particles having a low coefficientof heat expansion control even the coefficient of heat expansion of thematrix and bring about an additional advantage that the overallcoefficients of heat expansion of the steels are made further smallerthan the calculated values.

Though the case where only chromium constitutes the principal alloyelement of the matrix has been explained above, such an alloy steel ispoor in thoughness at a room temperature and yet has not so highstrength at a high temperature as compatible with the excellentresistivity to corrosion at high temperatures as stated above.Therefore, when a steel is required also to possess these qualities, thematrix is preferable to be made to be austenite dome with at least oneelement selected from the group of nickel and manganese, and, also it iseffective that the matrix is made with at least one element selectedfrom the group of silicon and molybdenum. Tables 5 and 6 show the effectof the above elements on strength at high temperatures, and Table 7shows that the steels shown in Table 6 combines an excellent corrosionresistivity at high temperatures with excellent strength at hightemperatures. Consequently, in order that the steel according to thepresent invention secures such high strength at high temperatures inaddition to corrosion resistivity at high temperatures as not obtainablein the conventional steels it is required that those elements are addedin the same total amount at the maximum as chromium to make the matrixan austenite structure, whereby their maximum amounts in total are 30percent by weight for the steel containing 30 percent by weight chromiumat the maximum.

TABLE 4 Range Coafiicient of 01 the heat-expansion Chemical composltion(percent) measured I tempara- Measured Calculated Sample C Si Mn N1 OrMo carbonitride tures, 0. value value Invented steel I 0. 05 2.1 2.1Ti-basel 3.2.. 25600 11.7)(10 12.3)(10- Invented steel II 0.0 1.7 2.2Ti-base) 7.5.. 25-600 11.0X10- 12.0X10- 0. 1.7 3. 5 (Zr-base) 4.7..25-600 11-.0X10' 12.1X10' Table Sample Room temperature High temperatureKind and amount of Tensile Elong Tensile Elong- Composition dispersedparticles strength ation Reduction strength ation Reduction Temp. ofmatrix (weight (kglmm of area(%) (kglmm (7c) of area(%) (C) 10%Cr (ZrNb)base 62 98.6 8.8 18.5 9.6 41.0 81.6 10%Cr 3%MO 800 2%Si (Zr Nb)base3.5 75.4 21.6 45.5 12.8 69.6 96.8 18%Cr (Ti)base 7.5 66.8 11.8 22.9 10.330.3 75.0 18%Cr 8%Ni 800 +1%Si (Ti)base 6.4 64.4 43.2 60.4 22.6 43.464.0 20%Cr (Ti v)base 4.7 70.3 10.3 21.0 3.4 93.0 97.9 1,000 20%Cr 20%Nl(Ti V)base 4.5 715 41.5 57.3 10.8 78.3 90.4

Table 6 Tensile strength( kg[mm"') Test Temperatures C) AlSl 202Invented steel* 'Note:- AlS1 202 4.5; by weight of Ti-cnrhonitride sultof the comparison of sulphur attack, oxidation resistances and leadattack resistance, as apparent from Table 8, it should rather superiorresistivities to the SUl-l 33B-steel, one of the best heat-resistingsteels already known in the field of art. Moreover, from the economicalpoint of view, since the present steel requires no valuable nickel andyet the chromium quantity is sufficient in less than one half of thatcontained in the SUH-33B, its manufacturing cost becomes less thanNote:- AlSl-ZOZ 4.5% by weight of Ti-carbonitridc The following specificexamples will serve to illustrate the excellence of the steels accordingto the present invention in their qualities and an economical adentinvention, a heat-resisting steel having better heat resistivities canbe obtained quite at a low price.

vantage as heat-resisting steels. 4O

EXAMPLE 1 EXAMPLE 2 h heat l y of the $1961 accordlng 9 Though theso-called 18-8 stainless steel is used in em mvemlon Whlch was Qbtamed yp R various fields as a heat-resisting stainless steel, the valuclhes 9p y y g CaYbQmmde, combmed able nickel in it was replaced by 5 percentby weight with tltan um and niobium 1n the ratlo 0f 3 I to the low costzirconium carbonitride, and the fine particles steel consisting mainlyof, by weight, 20 percent silicon, of the carbonitride were uniformlydispersed in the 18 10 percent chromium and 3.0 percent molybdenum ispercent-chromium steel. In this steel according to the shown in Table 8in comparison with the SUl-l 33B- present invention the matrix wasferrite as shown in steel. Table 9, so it could eliminate the defects ofthe 18-8 Table 8 Test items Test condition Invented steel SUH 33BS-attacking rate 1n so 800C 0.05 0.03 (mg/cm /hr) do. 900C 0.17 0.25Oxidation rate In the air 800C 0.00 0.00 (mglcm /hr) do. 900C 0.03 0.01Pb-attacking rate In PbO 915C 950 780 (initially) (mg/cm /hr) do. 915C440 (finally) Since the steel according to the present inventioncontained 10 percent by weight chromium, its usability covers atemperature range up to 900C, and as the restainless steel caused by itsbeing an austenite steel, and at the same time it showed theextraordinarily excellent resistivity in the oxidation and the sulphurattack at a temperature above 900C. Accordingly, the present steel maybe used, under some circumstances. as a superior and yet cheapheat-resisting stainless steel in place of a costly 18-8 stainlesssteel.

Table 9 Test item Test condition Invented steel 188 Stainless Steelobserved cracking in lntergranular corrosion cracking By 118-4304 notrecognized 3 pieces among 10 pieces Coefficient of heat expansion (X10-) 25 600C 11.0 1708 V-attack resistance (loss in weight-mg/cm) 1nV205 800C X 20 hr 48 45 Pb-attack resistance (loss in weight-mg/cm") 1nPbO 915C x 1 hr 1.000 1,050 Oxidation resistance In the air 900C X 10 hr0.0 0.0 (increase in weight-mg/cm-") do. 1,000C X 10 hr 2.1 23.4S-attack resistance in 100% S 800C X hr 0.5 0.3 (increase inweight-mg/cm) do. 900C X 10 hr 1.2 23.6 do. 1,000"c x 10 hr 2.8 146EXAMPLE 3 when immersed in lead oxide at 915C) A thermal reactor for theexhaust of automobiles demands a heat-resisting steel which can standthe high initially: 500 mglcmz/hr temperature gas above 1,000 C, but thehitherto subsequently: 15o mg/cm lhr known heat-resisting steels havenot met this desire in such points as the resistivity against sulphuretc. Ac- D. The momentary tensile strength and elongation at cording tothe present invention, a steel which can an- 1,000C: swer such a desirecould be manufactured by dispersing 14.3 kg/mm and 64.0 percent,respectively. about 5 percent by weight titanium carbonitride into Whatis Claimed is: the steel consisting mainly of, by weight, 25 percent l.A heat-resisting nickel-free steel consisting essenchr i 20 r t ick l ad 2 percent l bd tially of a uniform dispersion of 2 percent to 10percent num. The experimental results of the steel were as foly welghtof 1110f? cafbonltl'ldes SeleCt6d f 1 the group consistmg of titamum,zlrconium, n1ob1um, A. The thickness of the oxidation layer when thesteel tantamm, and Va nadlum Carbomtrides, P y was repeatedly subjectedto the heating and the sudden Welght of Chromlum and to 30 Percent yWelghl of cooling cycle f LQOOOC X 1 hour in the one or more elementsselected from the group conslstin the r Cycle 000038 mm ing of boron,silicon, molybdenum, manganese, copper in the respective subsequentcycle (on the average) and and the balance bemg and unavold' 0.00004mm/cycle able lmpurltles- B. The thickness of the sulphurated layer whensub- 40

1. A HEAT-RESISTING NICKEL-FREE STEEL CONSISTING ESSENTIALLY OF AUNIFORM DISPERSION OF 2 PERCENT TO 10 PERCENT BY WEIGHT OF ONE ORE MORECARBONITRIDES SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZICRONIUM,NIOBIUM, TANTALUM, AND VANADIUM CARBONITRIDES, 1-30 PERCENT BY WEIGHT OFCHROMIUM AD 0.2 TO 30 PERCENT BY WEIGHT OF ONE OR MORE ELEMENTS SELECTEDFROM THE GROUP CONSISTING OF BORON, SILICON, MOLYBDENUM, MANGANESE,COPPER AND ALUMINUM AND THE BALANCE BEING IRON AND UNAVOIDABLEIMPURITIES.